Echo ultrasound techniques are a popular modality for imaging structures within the human body. One or more ultrasound transducers are utilized to project ultrasound energy into the body. The energy is reflected from impedance discontinuities associated with organ boundaries and other structures within the body; the resultant echoes are detected by one or more ultrasound transducers (which may be the same transducers used to transmit the energy). Detected echo signals are processed, using well known techniques, to produce images of the body structures. In one such technique, a narrow beam of ultrasound energy is scanned across the body to provide image information in a body plane.
A beam of ultrasound may be scanned across a body by sequentially activating individual ultrasound transducer elements in a linear array of such elements. Apparatus of this type is described, for example, in the article Medical Ultrasound Imaging: An Overview of Principles and Instrumentation, J. F. Havlice and J. C. Taenzer, Proceedings of the IEEE, Vol. 67, No. 4, April 1979, page 620 and in the article Methods and Terminology for Diagnostic Ultrasound Imaging Systems, M. G. Maginness, page 641 of the same publication. Those articles are incorporated by reference herein as background material.
Efficient coupling of ultrasound energy from a transducer or array of transducers to a body or other object undergoing examination requires that the acoustic impedance of the transducer be matched to that of the test object. Ultrasound transducers typically used in medical applications comprise ceramics having an acoustic impedance of approximately 30.times.10.sup.6 kg/M.sup.2 sec. Human tissue has an acoustic impedance of approximately 1.5.times.10.sup.6 kg/M.sup.2 sec; thus an impedance matching structure is usually required between transducer ceramics and human tissue. Quarterwave matching windows, for example of the type described in U.S. patent application Ser. No. 104,516 filed on or about Dec. 17, 1979 (now abandoned), are commonly used for this purpose.
Wideband ultrasound pulses are typically utilized in medical imaging apparatus. Ideally, an impedance matching structure which couples pulses from the transducer to the human tissue should have a Gaussian frequency response as illustrated in FIG. 1. However, theoretical and experimental studies have shown that if a transducer is backed with air, a single quarterwave matching window will produce a double peaked frequency response of the type illustrated in FIG. 2. The prior art has recognized that a frequency response characteristic which approaches the ideal Gaussian may be achieved with an impedance matching structure comprising two or more quarterwave matching layers in cascade (that is one overlaying the other). The production of cascade matching structures of this type requires precise control of the layer thickness. Although such structures may be produced on experimental transducer arrays which are constructed from precision ground ceramic plates of uniform thickness, they are impractical for economical production of transducers which are generally formed from cast ceramic plates and which may warp or have varying thickness.
U.S. Pat. No. 4,326,418 represents another prior art solution to the impedance matching problem. That application describes an impedance matching structure having periodic, staircase-like thickness variations which effectively produce a Gaussian frequency response. While highly effective, the impedance matching structure described therein is relatively expensive to produce since either the periodic structure or the dies from which it is cast must be produced by a large number of precision machining operations.